1. Field of the Invention
The present invention relates to an apparatus and method for locking screws so as to capture them and prevent their loosening. The invention has particular application to the remote underwater replacement and locking of the screws, such as those which secure the baffle plates to the former plates or the former plates to the core barrel in the internal region of a nuclear reactor core.
2. Description of the Prior Art
In a nuclear reactor the core assembly is disposed within a cylindrical core barrel and, more particularly, is confined within a zig-zag array of baffle plates which are spaced inwardly from the core barrel side wall. The space between the baffle plates and the core barrel side wall is closed by a plurality of vertically spaced-apart former plates. The baffle plates are secured to the former plates by a plurality of locking screw assemblies. Typically, the screws are received through complementary bores in the baffle plate and threadedly engaged with the former plate, the screw head being recessed in a counterbore portion of the screw bore. The screw heads are generally lock welded in place on the baffle plate.
Occasionally, the baffle plate screws become distressed in use and must be removed and replaced. Such replacement is complicated by the fact that the baffle plates are now underwater in an irradiated environment. Therefore, remote replacement of the screws is essential. Because the work is done underwater, the replacement screws cannot be readily lock welded in place, as were the original screws. Furthermore, the environment does not permit extensive machining of the baffle plates. Nevertheless, it is desirable that the replacement bolts be locked in place with a so-called "Class A locking device", i.e., one which not only prevents loosening of the screw, but also captures the screw head and prevents it from becoming a loose part in the event that the screw shank should break.
At present, no satisfactory technique exists for locking replacement screws in place with a Class A locking device. Prior locking arrangements for screw or bolt assemblies are known, but these prior arrangements include a portion to be folded up against a nut or screw head after tightening of the screw or bolt. Such arrangements require lateral access to the locking device to effect the folding or bending operation. Therefore, these arrangements would not be suitable for use in applications where the screw head is recessed in the counterbore.
One type of screw locking technique which has been considered involves the machining of an annular groove around the counterbore of the screw bore, leaving a thin web of baffle plate material separating the groove from the counterbore portion of the screw bore. A staking tool would then be inserted in the groove to deform the web material into a notch or recess in the screw head. Such an approach has several disadvantages. First of all, it involves significant machining of the surface of the baffle plate, which frequently becomes embrittled due to reactor core exposure. Furthermore, if a mistake is made in the machining operation, it would be very difficult to redo or repair.